The invention concerns a process and apparatus for the preparation of color recipes. More specifically, the present invention relates to a process and apparatus for preparing a color formula for the reproduction of the color of a master by mixing suitable concentrations of colorants, in particular colorings and pigments, wherein the master is exposed by a source of directional measuring light, and the light directionally reflected by the master is passed after spectral decomposition to a detector which produces spectral measuring data from the reflected light. The measured data are converted by a computer unit into concentrations of colorants having known optical data previously determined by spectrophotometric analysis via calibrated colorations using known standard colorant concentrations. A sample, adjusted in accordance with the colorant concentrations calculated spectrophotometrically, is compared with the master, and the colorant concentrations corrected in keeping with the differences measured, this correction process being repeated until the difference measured is less than a predetermined minimum color deviation.
The preparation of color recipes, i.e. the calculation of the appropriate concentrations of colorants or pigments, to reproduce the color of a master, is one of the most important tasks in the industrial processing of colorants. In particular in the textile, printing, automotive and plastic industries, requirements relative to the accuracy and reproducibility of colors are high. For this reason colorimetric measurements represent an exceedingly useful and indispensable instrument in the quality control of colored products and the colorants required (colors, pigments) and the preparation of color recipes.
In an article by Dr. Ludwig Gall in "Farbe und Lack", Vol. 80, No. 4, 1974, pages 297-306, the principal process for the preparation of color recipes is described. The master to be reproduced relative to color is spectrophotometrically analyzed and the measured data determined converted in a computer unit into concentration proportions of colorants (colors, pigments) of known optical data, such as for example absorption and scatter coefficients. The quality and accuracy of the colorant concentrations determined depends on the accuracy of the knowledge of the optical data of the colorants available. For this purpose, standard colorings are prepared for each colorant with different, accurately known colorant concentrations and analyzed by spectrophotometry. The wavelength dependent optical data (absorption and scatter coefficient) of each colorant as a function of the colorant concentration are stored in a memory unit for use in the calculation of colorant concentration for the master to be reproduced in color.
The surface state of the sample (this could be a calibration coloring, a master or a reproduction) significantly affects the measured results and thus the optical data and the colorant concentrations determined. These boundary surface effects of the boundary layer between surrounding media and the sample surface are known generally as gloss effects and become apparent as a more or less glossy or matte surface of the sample. In the integrating-sphere measuring geometries usually employed (for example d/8, 8/d, . . . ; the number or symbol preceding the slash refers to the angle enclosed between the incident light and a normal onto the sample surface, while the number or symbol following it designates the angle between the reflected light and the normal; d signifies diffuse), the entire gloss component of the reflected light is included. This gloss component in the heretofore most frequently used color formulation method according to the Kubelka-Munk two-constant theory with the inclusion of the Saunderson approximation (article by Dr. Ludwig Gall in "Farbe und Lack", Vol. 80, No. 4, 1974, pages 297-306 and "Practical Color Measuring Course", Bundesanstalt fur Materialpru fung [Federal Material Testing Institute], DK 535.64/.65, 1982 Edition, pages 53-57) is taken into account in that depending on the surface characteristics (a distinction may be made for example between glossy, semigloss or matte surface classes) of the master to be matched, color formulation is carried out with reference to an appropriate set of optical data, from glossy, semigloss or matte calibrating colorations. It is also necessary to determine a separate wavelength dependent optical data set for each type of surface characteristic. This requirement results in an enormous preparatory analysis effort for all available colorants, as in addition, every calibrating and measuring process is repeated up to five times or more for every surface class of the calibrating colorations, in order to obtain adequate statistical certainty. Processes based essentially on the approximations of Kubelka-Munk and Saunderson are described for example in U.S. Pat. No. 3,601,589 and in JP-A-62-90518. In EP-A-065,484 and DE-A-1,547,467, respectively, a reflectance color measuring instrument and a photoelectric color brightness comparison instrument are described, in which direct surface reflections are blocked out by crossed polarizers.
In the known measuring devices highly different measuring geometries are employed. In a publication in "Color research and application", Vol. 13, No. 2, Apr. 1988, pages 113-118, Danny C. Rich describes the effect of the measuring geometry on color matching. As the result of a comparative experiment between integrating-sphere geometries and bidirectional geometries, in particular 0/45 geometries, such as those described for example in the German standard DIN 5033, Part 7, Jul. 1983, he came to the conclusion that the 0/45 geometry (and thus the equivalent 45/0 geometry) is superior to the integrating-sphere geometry (d/8). As shown by these comparative experiments by Rich, the different sample surface characteristics may be taken into account by means of separate surface dependent optical data sets of colorants, in bidirectional measuring processes also.
A first attempt to move away from this enormous preparatory analytical effort, is represented by a calculation method based on the three-beam theory of Dr. H. Pauli and Dr. D. Eitle (Colour 73, Adam Hilger, London 1973, pages 423 to 426 and a preprint of a pertinent paper presented at the FATIPEC XIV Congress, Budapest 1978, pages 209-213), in which the gloss component is taken into account empirically in the calculation of the colorant concentration, and therefore is able to work with a single surface state dependent set of optical data (wavelength dependent), independently of the surface characteristics of the colored master to be matched. However, it has been necessary heretofore in this process also to determine the entire gloss component, both in the analysis of the colored master and in the determination of the optical data of the colorants, by means of an involved integrating-sphere measuring geometry.
The above described processes for the preparation of color formulations on the one hand have the disadvantage of a high preparatory analytical effort of the colorants, with wavelength dependent optical data sets of the colorants differing as a function of surface characteristics and of different measuring layouts and depending on the type of measuring geometry. On the other hand, integrating spheres with the usual diameters of up to about 20 cm and more, together with the associated peripherals (computer unit, etc.) are very cumbersome and heavy and may be located stationarily in a central laboratory only. There is, however, a need to have available a mobile measuring apparatus and process for "on site" measurements and color formulations.